The invention relates generally to timing phase recovery, and more particularly to timing phase recovery in digital communications systems.
Generally, in a synchronous digital communication system, digital information is extracted from a received signal by demodulation and then sampling. In order to accurately sample the demodulated signal, usually some level of signal synchronization is required. Loosely speaking, synchronization is the function of determining some of the characteristics of the received signal. These characteristics are used to extract the digital information from the received signal. Synchronization is often referred to as symbol synchronization or timing recovery.
Timing recovery is an important function of a synchronous digital communication system. A receiver in the synchronous digital communication system must not only know the frequency of the received symbols, but also when to take samples within the symbol interval.
The choice of when to take samples within the symbol interval is known as the timing phase. Timing phase recovery is the task of estimating the timing phase of the received signal. Once the timing phase is estimated, the sampling time is adjusted to maximize the performance of the receiver. This performance maximization often includes adjusting the sampling instant to the maximum open-eye position of the demodulated signal. This corresponds to sampling the received signal in the centre of each symbol interval.
In known mobile digital communication systems, often the clock of the mobile unit is synchronized to a clock in the base station receiver. Therefore, in this implementation, the base station receiver only compensates for the propagation delay, carrier frequency offset and fading of the received signal when recovering the timing phase. Many known systems, however, are not robust under Doppler fading, carrier frequency offset and low C/I conditions.
Clearly, there is a need to robustly estimate the timing phase for a received signal in a digital communication system.
The invention is directed to a method and apparatus for robustly estimating the timing phase for a received signal in a digital communication system.
An aspect of the invention is a method for timing phase recovery including the following steps. First, estimating an open-eye sequence of a received signal. Second, estimating a channel response using the open-eye sequence and an adjusted signal. Third, correlating the estimated channel response with an ideal channel response to determine a peak correlation location. The peak correlation location corresponds to the recovered timing phase.
Another aspect of the invention is a system for timing phase recovery including means for estimating an open-eye sequence of a received signal, a channel estimator and a correlator. The channel estimator is used for estimating a channel response using the open-eye sequence and an adjusted signal. The correlator is used for correlating the estimated channel response with an ideal channel response to determine a peak correlation location, the peak correlation location corresponding to the recovered timing phase.
An advantage of the invention is robustness to Doppler fading, carrier frequency offset and low C/I.
Other aspects and advantages of the invention, as well as the structure and operation of various embodiments of the invention, will become apparent to those ordinarily skilled in the art upon review of the following description of the invention in conjunction with the accompanying drawings.